1. Field of the Invention
The present invention relates to a zinc oxide thin film, a process for producing the zinc oxide thin film, a photoelectric conversion element having the zinc oxide thin film, and a process for producing the photoelectric conversion element. More particularly, the invention relates to a zinc oxide thin film having formed a considerably uneven surface shape with the optical confinement effect so as to be increased in the texture level, a photoelectric conversion element having the zinc oxide thin film, and processes for producing them.
2. Related Background Art
The photoelectric conversion elements having a semiconductor layer comprised of hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated amorphous silicon carbide, microcrystal silicon, polycrystal silicon, or the like have been utilized heretofore with a reflective layer being provided on a back surface of the semiconductor layer in order to improve the collection efficiency at long wavelengths. Such a reflective layer should desirably demonstrate effective reflection characteristics at wavelengths near the band edge of the semiconductor materials where absorption thereof is small, i.e., at 800 nm to 1200 nm.
Materials adequately satisfying this condition are metals such as gold, silver, copper, and aluminum, and alloys thereof.
Further, another technology is also employed which provides an optically transparent layer with an unevenness in a predetermined wavelength range to attain optical confinement. Generally, an attempt has been made to provide between the aforementioned metal layer and the semiconductor layer, a transparent electroconductive layer having an unevenness to effectively utilize reflected light, thus improving the short circuit current density Jsc.
In addition, the transparent electroconductive layer prevents decreases in performance characteristics due to shunt paths.
Further, in order to effectively utilize incident light to improve the short circuit current density Jsc, there is also another attempt to provide the transparent electroconductive layer having an unevenness on the light incidence side of the semiconductor layer to increase path lengths of the incident light in the semiconductor layer.
Most generally, these layers are formed by methods such as vacuum vapor deposition and sputtering and shows an improvement of 1 mA/cm.sup.2 or more in terms of the short circuit current density Jsc.
Examples thereof are Prior art 1: "29p-MF-22: Optical confinement effect in a-SiGe solar cells on stainless steel substrate," Extended Abstracts (The 51th Autumn Meeting, 1990); The Japan Society of Applied Physics, p. 747, Prior art 2: "P-IA-15 a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells With Bandgap Profiling," Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p. 381, 1990, and so on, which describe the reflectivity and texture structure of a reflective layer comprised of silver atoms.
These examples describe that forming the reflective layer by deposition of two silver layers at different substrate temperatures to form an effective unevenness, accomplished an increase in the short circuit current by the optical confinement effect by a combination thereof with a zinc oxide layer.
On the other hand, "Electrolyte Optimization for Cathodic Growth of Zinc Oxide Films" M. IZAKI and T. Omi, J. Electrochem. Soc., Vol. 143, March 1996, L53 and Japanese Patent Application Laid-Open No. 8-217443 describe a method for uniformly making a zinc oxide film with excellent transmittance as the aforementioned transparent electroconductive layer by electrolysis of an aqueous solution containing zinc ions and nitric ions.
The transparent electroconductive layers produced by vacuum vapor deposition or sputtering, disclosed heretofore as described above, had excellent photoelectric conversion characteristics, but, because they were the zinc oxide thin films having the crystal structure of the c axis orientation according to the law of Bravais, forming the transparent electroconductive layer having the unevenness structure necessitated preparation at a high temperature or increase in thickness.
No attempt has been made heretofore yet to deposit by electrolysis the zinc oxide thin film provided with the uneven shape exhibiting good optical confinement effect.
Thus, the present invention has been accomplished to solve the above issue and an object of the present invention is to provide a zinc oxide thin film with an increased texture level and improved short circuit current density (Jsc) but without the need for the preparation under high-temperature conditions nor for an increase in thickness, and to provide a photoelectric conversion element comprised of the zinc oxide thin film.